In putting the 2-story addition on the right side of the house instead of the left side, I had to do some reconfiguring of the stairways to allow for doors into those side rooms.  On the first floor, I flipped the direction of the stairs and this meant I had to cut a new hole opposite the precut one on the second floor.

In order to have enough floor space to create a doorway into the second floor side room, I had to fill in the old hole.  The best way I could think of doing this was to create floor joists from strip wood to support two pieces of flat bass wood board cut to the shape of the hole.  On the front edge of the opening, I used a piece of 1/8" x 3/8" strip wood to create an aesthetic finish.

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I cut two pieces in the shape of the opening out of 1/16" thick bass wood board.

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I used 1/4" x 1/4" strip wood to create the support beams.

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I glued the joists in place and then glued the bass wood pieces on either side.

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The flooring on the second floor would mask the patch from above, but on the underside -- or first floor ceiling -- I had to spackle and paint to match the rest of the ceiling.  Here's the patch after spackling but before painting.

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